Rational Design of an Ultrasensitive and Highly Selective Chemodosimeter by a Dual Quenching Mechanism for Cysteine Based on a Facile Michael-Transcyclization Cascade Reaction

A novel dual-function fluorescent probe for the detection of cysteine and its applications in vitro

A fluorescent probe of both colorimetric and ratiometric type for highly selective and sensitive detection of Cys (cysteine) is very important in biological analysis. In this work, a new colorimetric and ratiometric fluorescent probe ((E)-2-(2-(5-(4-(acryloyloxy)phenyl)furan-2-yl)vinyl)-3-methylbenzo[d]thiazol-3-ium iodide, LP-1) was designed and synthesized for the detection of Cys. The reaction mechanism of LP-1 toward Cys involves a conjugate addition reaction between Cys and the α,β-unsaturated carbonyl group, leading to the formation of an intermediate thioether, followed by intramolecular cyclization to produce the desired compounds LP-1-OH. At this point, the ICT process is activated, significantly increasing the fluorescence intensity of the molecules. Meanwhile, LP-1 is highly selective and sensitive to Cys identification under optimized experimental conditions. LP-1 shows a good linear relationship in the range of Cys concentration from 0.40 μM to 40 μM (R2 = 0.9942) and the limit of detection (LOD) of Cys is 0.19 μM. In addition, we have developed a simple, portable and low-cost smartphone-based high-sensitivity Cys detection method based on naked eye obvious color detection. LP-1 also has low cell toxicity and can be successfully used for biological imaging of Cys, suggesting that it is a promising biological application tool for Cys detection.

Improving the stability of thiol-maleimide bioconjugates via the formation of a thiazine structure

Linker stability is critically important for the efficacy and safety of peptide and protein conjugates used for biological applications. One common conjugation strategy, thiol-maleimide coupling, generates a succinimidyl thioether linker with limited stability under physiological conditions. We have shown in previous work that when a peptide with an N-terminal cysteine is conjugated to a maleimide reagent, a thiazine structure is formed via a chemical rearrangement. Our preliminary work indicated that the thiazine linker has favorable stability. Here, we report the evaluation of a thiazine linker as an alternative to the widely used succinimidyl thioether linker for thiol-maleimide bioconjugation. The stability of the thiazine conjugate in comparison to the thioether conjugate was assessed across a broad pH range. Additionally, the propensity for retro-Michael reaction and cross-reactivity with other thiols was evaluated by treating conjugates in the presence of glutathione. The studies indicated that the thiazine linker degrades markedly slower than the thioether conjugate. In addition, the thiazine linker is over 20 times less susceptible to glutathione adduct formation. The NMR study of the thiazine structure confirmed that the formation of the thiazine linker is a stereoselective process that yields a single diastereomer. In summary, we propose the use of the thiazine linker obtained by conjugation of maleimide-containing reagents with peptides or proteins presenting an N-terminal cysteine as a novel approach for bioconjugation. The advantages of this approach are the formation of a linker with a well-defined stereochemical configuration, increased stability at physiological pH, and a strongly reduced propensity for thiol exchange.

Sequence sensitivity and pH dependence of maleimide conjugated N-terminal cysteine peptides to thiazine rearrangement

Thiazine formation during the conjugation of N‐terminal cysteine peptides to maleimides is an underreported side reaction in the peptide literature. When the conjugation was performed at neutral and basic pH, we observed the thiazine isomer as a significant by‐product. Nuclear magnetic resonance (NMR) spectroscopy confirmed the structure of the six‐membered thiazine and ultra‐high performance liquid chromatography (UHPLC) combined with tandem mass spectrometry (MS/MS) allowed for facile, unambiguous detection due to a unique thiazine mass fragment. Furthermore, substitution of various amino acids adjacent to the N‐terminal cysteine in a tripeptide model system resulted in different rates of thiazine formation, albeit within the same order of magnitude. We also determined that varying the N‐substitution of the maleimide affects the thiazine conversion rate. Altogether, our findings suggest that thiazine rearrangement for N‐terminal cysteine‐maleimide adducts is a general side reaction that is applicable to other peptide or protein systems. Performing the conjugation reaction under acidic conditions or avoiding the use of an N‐terminal cysteine with a free amino group prevents the formation of the thiazine impurity.

Ultrasensitive and selective fluorescent sensor for cysteine and application to drug analysis and bioimaging

A fluorescent sensor based on coumarin-maleimide conjugate was developed for efficient discrimination of Cys from Hcy and GSH in both organic and aqueous solution. Addition of Cys to the non-fluorescent sensor solution in DMF induced bright blue fluorescence and enhanced the fluorescence intensity by 320-fold while other amino acids and biothiols (Gly, Hcy, GSH, Glu, Val, Tyr, Arg, Trp, Lys, His, Leu, Phe, Asp and Met) did not bring about remarked change. The sensor responds to Cys extremely rapidly. If Cys was added to the sensor solution, the fluorescence intensity increased by 170-fold immediately and attained the maximum value in 5 min. A linear relationship was observed between Cys concentration within 2-20 μM and the fluorescence intensity of the sensor solution. The detection limit of the sensor toward Cys is as low as 4.7 nM. The sensor is also effective for specific detection of Cys in aqueous (DMF/H₂O = 9:1, v/v) solution. Practical application of the sensor to drug analysis and bioimaging of living Hela cells has been verified. Possible sensing mechanism of the sensor toward Cys has been proposed.

A rapid “on–off–on” mitochondria-targeted phosphorescent probe for selective and consecutive detection of Cu2+ and cysteine in live cells and zebrafish

The detection of mitochondrial Cu²⁺ and cysteine is very important for investigating cellular functions or dysfunctions. In this study, we designed a novel cyclometalated iridium(iii) luminescence chemosensor Ir bearing a bidentate chelating pyrazolyl-pyridine ligand as a copper-specific receptor. The biocompatible and photostable Ir complex exhibited not only mitochondria-targeting properties but also an “on–off–on” type phosphorescence change for the reversible dual detection of Cu²⁺ and cysteine. Ir had a highly sensitive (detection limit = 20 nM) and selective sensor performance for Cu²⁺ in aqueous solution due to the formation of a non-phosphorescent Ir–Cu(ii) ensemble through 1 : 1 binding. According to the displacement approach, Ir was released from the Ir–Cu(ii) ensemble accompanied with “turn-on” phosphorescence in the presence of 0–10 μM cysteine, with a low detection limit of 54 nM. This “on–off–on” process could be accomplished within 30 s and repeated at least five times without significant loss of signal strength. Moreover, benefiting from its good permeability, low cytotoxicity, high efficiency, and anti-interference properties, Ir was found to be suitable for imaging and detecting mitochondrial Cu²⁺ and cysteine in living cells and zebrafish.

An iridium(iii) complex-based mitochondria targeting phosphorescent probe for selectively detecting Cu²⁺ and Cys in aqueous solution, living cells and zebrafish has been developed.

The chronological evolution of small organic molecular fluorescent probes for thiols

Abnormal concentrations of biothiols such as cysteine, homocysteine and glutathione are associated with various major diseases. In biological systems, the structural similarity and functional distinction of these three small molecular thiols has not only required rigorous molecular design of the fluorescent probes used to detect each thiol specifically, but it has also inspired scientists to uncover the ambiguous biological relationships between these bio-thiols. In this minireview, we will discuss the evolution of small organic molecular fluorescent probes for the detection of thiols over the past 60 years, highlighting the potent methodologies used in the design of thiol probes and their particular applications in the semi-quantification of cellular thiols and real-time labelling. At the same time, the present challenges that limit their further application will be discussed. We hope that this minireview will promote future research to enable deeper insight into the crucial role of thiols in biological systems.

The chronological evolution of small organic molecular fluorescent probes for thiols: from separation dependency analysis to cellular specific analysis, what's next?

Improving the Stability of Maleimide-Thiol Conjugation for Drug Targeting

Maleimides are essential compounds for drug conjugation reactions via thiols to antibodies, peptides and other targeting units. However, one main drawback is the occurrence of thiol exchange reactions with, for example, glutathione resulting in loss of the targeting ability. A new strategy to overcome such retro-Michael exchange processes of maleimide-thiol conjugates by stabilization of the thiosuccinimide via a transcyclization reaction is presented. This reaction enables the straightforward synthesis of stable maleimide-thiol adducts essential in drug-conjugation applications.

Bioluminescence imaging of exogenous & endogenous cysteine in vivo with a highly selective probe

Cysteine (Cys) is a semi-essential amino acid that exerts a vital role in numerous biological functions. A noninvasive method for in vivo imaging of cysteine could represent a valuable tool for research cysteine and its complex contributions in living organisms. Thus, we developed a turn-on bioluminescence probe (CBP) not only for detecting exogenous and endogenous cysteine in vitro and in vivo, but also for visualizing these cysteines in whole animal. The current applications may help shed light on the complex mechanisms of cysteine in miscellaneous physiological and pathological processes.

A rhodol-enone dye platform with dual reaction triggers for specific detection of Cys

In this work, we have developed a rhodol-enone dye platform with bis-reaction-triggers for the selective detection of Cys.

A dihydronaphthalene based fluorescence probe for sensitive detection of cysteine and its application in bioimaging

A novel small molecule fluorescent probe NC-Cys for monitoring cysteine based on dihydronaphthalene was developed.

An enhanced fluorescence sensor for specific detection Cys over Hcy/GSH and its bioimaging in living cells

Cysteine (Cys) is not only the central matter of sulfur metabolism in cells but also the only amino acid with reduced thiol group in 20 kinds of natural amino acids. In animal cells, Cys is taking part in many important and essential biological functions including protein synthesis, detoxification and metabolism. The development and application of fluorescent probes for the detection of Cys have attracted more and more attention and interest. Herein, we report a new fluorescent probe NFA that utilized naphthyl carboxy fluorescein as fluorophore and acryloyl group as reaction site for Cys specific detection. The probe essentially has weak fluorescence. Cys addition to NFA containing system induced distinct enhanced fluorescence emission which was attributed to the nucleophilic reaction of cysteine and acryloyl to release the fluorophore. The signal fluorescent response detection system allows NFA to be a reliable tool for Cys detection with low detection limit (0.58 μM). And NFA has been successfully applied for Cys imaging specifically in live Hela cells, which promotes the probe as a potential tool to understand the pathology of Cys related diseases.

Functional synthetic probes for selective targeting and multi-analyte detection and imaging

In contrast to the classical design of a probe with one binding site to target one specific analyte, probes with multiple interaction sites or, alternatively, with single sites promoting tandem reactions to target one or multiple analytes, have been developed. They have been used in addressing the inherent challenges of selective targeting in the presence of structurally similar compounds and in complex matrices, as well as the visualization of the in vivo interaction or crosstalk between the analytes. Examples of analytes include reactive sulfur species, reactive oxygen species, nucleotides and enzymes. This review focuses on recent innovations in probe design, detection mechanisms and the investigation of biological processes. The vision is to promote the ongoing development of fluorescent probes to enable deeper insight into the physiology of bioactive analytes.

A long wavelength emission two-photon fluorescent probe for highly selective detection of cysteine in living cells and an inflamed mouse model

As a semi-essential proteinogenic amino acid and biothiol, cysteine (Cys) is highly important in many basic cellular processes.

Recent Advances in the Development of Optical Imaging Probes for γ-Glutamyltranspeptidase

γ-Glutamyltranspeptidase (GGT) is a cell-membrane-bound protease that participates in cellular glutathione and cysteine homeostasis, which are closely related to many physiological and pathological processes. The accurate measurement of GGT activity is useful for the early diagnosis of diseases. In the past few years, many efforts have been made to build optical imaging probes for the detection of GGT activity both in vitro and in vivo. In this Minireview, recent advances in the development of various optical imaging probes for GGT, including activatable fluorescence probes, ratiometric fluorescence probes, and activatable bioluminescence probes, are summarized. This review starts from the instruction of the GGT enzyme and its biological functions, followed by a discussion of activatable fluorescence probes that show off-on fluorescence in response to GGT. GGT-activatable two-photon fluorescence imaging probes with improved imaging depth and spatial resolution are also discussed. Ratiometric fluorescence probes capable of accurately reporting on GGT levels through a self-calibration mechanism are discussed, followed by describing GGT-activatable bioluminescence probes that can offer a high signal-to-background ratio to detect GGT in living mice. Finally, current challenges and further perspectives for the development of molecular imaging probes for GGT are addressed.

Bifunctional Fluorescent Probe for Sequential Sensing of Thiols and Primary Aliphatic Amines in Distinct Fluorescence Channels

Thiols and primary aliphatic amines (PAA) are ubiquitous and extremely important species in biological systems. They perform significant interplaying roles in complex biological events. A single fluorescent probe differentiating both thiols and PAA can contribute to understanding the intrinsic inter-relationship of thiols and PAA in biological processes. Herein, we rationally constructed the first fluorescent probe that can respond to thiols and PAA in different fluorescence channels. The probe exhibited a high selectivity and sensitivity to thiols and PAA. In addition, it displayed sequential sensing ability when the thiols and PAA coexisted. The application experiments indicated that the probe can be used for sensing thiols and PAA in human blood serum. Moreover, the fluorescence imaging of endogenous thiols and PAA as well as antihypertensive drugs captopril and amlodipine in living cells were successfully conducted.

A NIR facile, cell-compatible fluorescent sensor for glutathione based on Michael addition induced cascade spirolactam opening and its application in hepatocellular carcinoma

A NIR fluorescence probe with NIR emission wavelength at 746 nm and high quantum yield of 0.36 was designed and synthesized to selectively detect GSH over Hcy and Cys in living systems.

A simple two-photon turn-on fluorescent probe for the selective detection of cysteine based on a dual PeT/ICT mechanism

Herein, a simple two-photon turn-on fluorescent probe, N-(6-acyl-2-naphthayl)-maleimide (1), based on a dual PeT/ICT quenching mechanism is reported for the highly sensitive and selective detection of cysteine (Cys) over other biothiols. The probe was applied in the two-photon imaging of Cys in cultured HeLa cells, excited by a near-infrared laser at 690 nm.

N-(6-acyl-2-naphthayl)-maleimide (1) is a simple two-photon fluorescent probe with selectivity for cysteine, based on a thiol-Michael-addition-transcyclization cascade and dual PeT/ICT quenching mechanism.

EGFR-targeting peptide-coupled platinum(IV) complexes

The high mortality rate of lung cancer patients and the frequent occurrence of side effects during cancer therapy demonstrate the need for more selective and targeted drugs. An important and well-established target for lung cancer treatment is the occasionally mutated epidermal growth factor receptor (EGFR). As platinum(II) drugs are still the most important therapeutics against lung cancer, we synthesized in this study the first platinum(IV) complexes coupled to the EGFR-targeting peptide LARLLT (and the shuffled RTALLL as reference). Notably, HPLC–MS measurements revealed two different peaks with the same molecular mass, which turned out to be a transcyclization reaction in the linker between maleimide and the coupled cysteine moiety. With regard to the EGFR specificity, subsequent biological investigations (3-day viability, 14-day clonogenic assays and platinum uptake) on four different cell lines with different verified EGFR expression levels were performed. Unexpectedly, the results showed neither an enhanced activity nor an EGFR expression-dependent uptake of our new compounds. Consequently, fluorophore-coupled peptides were synthesized to re-evaluate the targeting ability of LARLLT itself. However, also with these molecules, flow cytometry measurements showed no correlation of drug uptake with the EGFR expression levels. Taken together, we successfully synthesized the first platinum(IV) complexes coupled to an EGFR-targeting peptide; however, the biological investigations revealed that LARLLT is not an appropriate peptide for enhancing the specific uptake of small-molecule drugs into EGFR-overexpressing cancer cells.

Fluorescein-Based Chromogenic and Ratiometric Fluorescence Probe for Highly Selective Detection of Cysteine and Its Application in Bioimaging

A dual mode fluorescent probe, which is based on an integration of fluorescein and coumarin fluorophores, was developed for the discrimination of Cys from Hcy and GSH. This probe (2) shows the advantage of quick reaction (5 min) with Cys, resulting in a strong fluorescence turn-on response when excited at 450 nm. Notably, it also demonstrates the ratiometric fluorescence property while excited by a shorter wavelength (332 nm). All of results suggest probe 2 has a high selectivity toward Cys even in the presence of other amino acids, cations and anions. The detection limit of Cys was calculated as 0.084 μM, which was much lower than the intracellular concentration. ¹H NMR, MS and DFT calculation were used to reveal the detection mechanism further. Finally, this low cytotoxic probe was successfully applied in bioimaging within HepG2 cells.

Orchestration of dual cyclization processes and dual quenching mechanisms for enhanced selectivity and drastic fluorescence turn-on detection of cysteine

Synergistic dual cyclizations and dual quenching mechanisms of acrylate and maleimide groups for improved turn-on fluorescence detection of cysteine.

Breaking the reduced glutathione-activated antioxidant defence for enhanced photodynamic therapy

Photodynamic therapy (PDT) has been applied in cancer treatment by utilizing reactive oxygen species (ROSs) to kill cancer cells.